Reverse osmosis units have been used for a number of decades to purify water for home, municipal and industrial uses. Examples of purification systems using reverse osmosis units to provide quantities of potable water suitable for home or other relatively limited uses are shown in the following patents of Donald T. Bray: U.S. Pat. Nos. 3,568,843; 3,794,172; 3,794,173 and 3,939,074. Such self-contained systems were generally designed to provide potable water on demand by being essentially permanently connected to a municipal water supply line pressure. They created a ready reservoir of purified or potable water which would be available to be dispensed to a user, either by gravity flow or by flow driven by the expansion of air compressed behind a bladder within a confined tank or the like.
Reverse osmosis membrane separation devices generally function on the basis of employing a significant fluid pressure difference between the inlet side of the membrane barrier, i.e., the feed side, and the opposite side of the membrane barrier, i.e., the product side. This pressure difference motivates the flow of fluid through the micropores of the membrane barrier while generally preventing the passage of salt and mineral solutes through the membrane. However, when such a membrane separation device is operated only intermittently, as is the practice with respect to most small scale household or domestic appliance versions of such reverse osmosis systems, a typical cyclic process occurs with every shutdown and start-up. Once the pressure difference across the membrane barrier is removed as a result of shutdown, flow through the membrane to the product side ceases. As a result, the two volumes of fluid which now occupy the void spaces on both sides of the membrane barrier are no longer subject to this pressure-driven osmotic separation effect. Consequently, the salt and mineral solutes in the liquid on the feed side of the membrane are now able to slowly pass through the membrane barrier and mix with the fluid on the product side, and the solutes generally continue to do so until equilibrated common solutions have been formed on both sides of the membrane.
When start-up again occurs so that high pressure and liquid flow are reestablished on the feed side of the membrane device, the osmotic separation process resumes; however, the residual volume of higher salt and mineral content liquid on the product side of the membrane in the purification unit will constitute the first portion of the product water stream to be discharged from the unit before a truly purified outlet stream will again be discharged from the purification unit. In most small scale or domestic appliance-type membrane separation systems, a liquid accumulation reservoir or tank is provided into which the discharge from the membrane purification unit is directed. Such a tank provides for rapid dispensing of a fairly large quantity of product liquid at the tap or other point of use, and because the tank or reservoir will be repeatedly filled to its desired capacity during periods of non-use, this arrangement permits the use of a small, economical membrane separation device having only a relatively low, direct, product flow rate. In such a system configuration, the problem of high salt and mineral content accumulating in the small volume of liquid on the product side within the purification unit during periods of shutdown becomes minimal because this small volume will be flushed into the tank or reservoir at the time of start-up where it becomes highly diluted with the very considerable volume of liquid within the product holding tank; as a result, the ultimate product remains generally acceptable.
However, there is some contamination of the product side liquid which is occurring and which, although acceptable, is still undesirable, and the search for new and improved water purification systems is an always continuing one.